1. Field of the Invention
The present invention relates to a fuel cell system and a control method of the fuel cell system.
2. Description of the Related Art
A fuel cell system having a fuel cell is known in the art. A reactant gas, or fuel gas (for example, hydrogen) is supplied to a fuel electrode in the fuel cell, and oxidizing gas (for example, air) is supplied to an oxidizer electrode in the fuel cell. The reactant gas and the oxidizing gas are electrochemically reacted with each other to generate power electrical power, as long as the fuel gas is being supplied. The electrochemical reaction of hydrogen (in the reactant gas) and oxygen (in the oxidizing gas) forms water vapor, some of which condenses to liquid water before being removed from the fuel cell system. A fuel cell system commonly includes a recirculation path for recirculating exhaust gas discharged from the fuel cell and combining the recirculating exhaust gas with reactant gas being supplied to the fuel cell, and a reactant gas recirculation pump to pump the recirculating exhaust gas through the reactant gas recirculation path.
A problem encountered in such fuel cell systems is that when the fuel cell system is stopped in a low-temperature environment, condensed water vapor can freeze. In particular, condensed water in the reactant gas recirculation path can freeze in the reactant gas recirculation pump, causing the pump to lock up and cease operating.
Prior art methods exist for preventing the reactant gas recirculation pump in the reactant gas recirculation path (i.e., the recirculation path for fuel gas) from being frozen and locked up in a low-temperature environment after operation of the fuel cell system is stopped. In a prior art method, after the fuel cell system is completely stopped, a system controller performs a two-step process, first controlling the recirculation pump to be rotationally driven at a low rotational speed when the recirculation pump temperature, as measured by a thermometer, becomes equal to or lower than a first threshold value, and then stopping the recirculation pump from being rotationally driven at the low rotational speed when the temperature becomes equal to or lower than a second threshold value. However, in the prior art methods, the rotational speed of the recirculation pump is controlled based solely upon the detection of the temperature of the recirculation pump, making it difficult to prevent condensation in the recirculation pump.